Due to a growing concern over greenhouse gas emissions, the development of technologies for separating carbon dioxide from gaseous process streams has gained in importance. In the future, power plants that generate electricity from coal, or other carbon-based fuels, may have to separate carbon dioxide from the gas stream. Chemical routes to hydrogen production from coal and natural gas rely on carbon dioxide separation that can be greatly simplified with the availability of high temperature carbon dioxide selective membranes. Solid oxide fuel cell (SOFC) designs that operate on carbon monoxide or hydrogen and carbon monoxide mixtures can benefit from the ability to separate carbon dioxide from the exhaust stream.
Methods conventionally used to separate carbon dioxide from a gas stream include chemical absorption using amine-based solvents or physical sorption using liquid or solid sorbents. However, these methods require that the gas mixture be at a temperature no higher than 100 degrees Celsius. Further, the energetic and economic penalties, incurred mostly from sorbent regeneration, are costly. At elevated temperatures, solid chemical absorbers, like lime, lithium zirconate, or lithium silicate, have been proposed but their use is complicated by slow kinetics and large material handling systems for solids.
Polymer and inorganic microporous membranes for carbon dioxide separation exist, but so far are limited by low selectivity or permeability, and low temperature operation. Particularly in gasification-based systems, it would be desirable to have carbon dioxide separation membranes that can operate in the temperature regime in which gasification or hot gas cleanup occurs. Attempts have been made to construct membranes from solid sorbents, such as lithium zirconate. However, these efforts so far have failed to produce viable membranes. Dense, dual-phase metal-carbonate membranes that operate by transporting carbon dioxide across as a carbonate ion in a molten carbonate phase, with a counter-current of electrons transporting in a metallic phase have also been proposed to produced membranes capable of operating at about 450-650 degrees Celsius. This technology, however, is limited by the requirement of having oxygen in the feed stream to convert the carbon dioxide (CO2) to carbonate (CO3), thus making it impractical for separation in fuel streams.
Mixtures of lithium carbonate and zirconia are known to react under low partial pressures of carbon dioxide to form lithium zirconate upon releasing gas phase carbon dioxide. The reaction can be reversed to utilize lithium zirconate as a chemical carbon dioxide absorption technology. As a result, lithium zirconate has recently been investigated as a membrane structure. However, selectivity of carbon dioxide over other gases was very poor, e.g., selectivity of carbon dioxide over methane was about five. In an economic evaluation of carbon dioxide removal from coal-fired flue gas streams, it has been estimated that in order to make membrane separation competitive with other carbon capture technologies, selectivity for carbon dioxide over other molecules should exceed 200.